Mar 10, - Many people get confused about when to use hyphens when writing ages. Here's a Quick and Dirty Tip that will help: When the age is an adjective that comes before the noun and modifies the noun, or when the age is a noun, hyphenate. My eight-year-old neighbor wrote a poem about commas for National Grammar Day. When should you hyphenate the phrase years old? Years-Old? twenty one year old I gave a lot If a noun comes after years old, you will want to hyphenate it.